1. Field of the Invention
The present invention relates to a moisture absorbent composition and a moisture absorbent molding, and in particular to a moisture absorbent composition and a moisture absorbent molding having a humidity conditioning function capable of holding humidity constant. The present invention further relates to a method for controlling the equilibrium humidity, and a method for controlling the equilibrium humidity maintaining time.
2. Description of Related Art
Moisture absorbents, such as silica gel, calcium chloride, quicklime, and zeolite, are used conventionally for preventing the quality deterioration of products due to oxidation or the like caused by moisture absorption in all fields of foodstuffs, pharmaceuticals, electronic components, precision machines, or the like. These moisture absorbents are wrapped with paper, nonwoven fabric, or the like or placed in a container in a granular or powdered state, and enclosed in packaging or the like with products.
On the other hand, for example, Patent document 1 (JP 07-53222B) and Patent document 2 (JP 07-96092B) propose to provide moisture absorbent compositions and moisture absorbent moldings in which specific moisture absorbents (for example, magnesium sulfate) are kneaded into thermoplastic resins to exhibit high moisture absorbency and water-holding capacity without bringing such drawbacks as scattering, moisture absorbency, and liquid leakage due to deliquescence.
Meanwhile, the development of various products is progressing according to diversification of needs for products, the advancement of manufacturing technologies, or the like, in recent years, and accordingly, there is a tendency to diversify the environment of product storage or the like. For example, cereals produce cracks or the like when moisture content is too low, and when moisture content is excessive, on the contrary, an enzymatic reaction progresses to cause deterioration in quality. Therefore, an environment capable of storing the cereals in an equilibrium humidity zone (approximately RH 50%) can be said to be a desirable storage environment. Further, products such as tea leaves, in which water is evaporated to some extent, deteriorate in quality through the enzymatic reaction or oxidation when moisture content increases due to moisture absorption. Therefore, an environment capable of storing the products in an equilibrium humidity zone (approximately RH 20%) is a desirable storage environment.
However, the moisture absorbents, such as silica gel, calcium chloride, quicklime and zeolite, which have been conventionally used as moisture absorbents, have strong desiccative capability and moisture absorbency according to the physical and chemical properties. When these moisture absorbents are put in sealed vessels, bags, and the like, the moisture absorbents absorbs internal moisture in a short period of time, and continue taking up the moisture infinitely until humidity reaches 0%. Thus, the conventional moisture absorbents did not have a humidity control function, and had a problem of incapability of responding to the storage of increasingly diversified products.
On the other hand, for example, Patent document 3 (JP 05-39379A) proposes a humidity conditioning composition and a humidity conditioning molding to which a humidity conditioning function is imparted by kneading a specific moisture absorbent (magnesium sulfate) into a thermoplastic resin.
In the case where magnesium sulfate is kneaded into a thermoplastic resin as a specific moisture absorbent, the composite of the resin and the magnesium sulfate brings about constant vapor pressure. However, the above Patent document 3 focuses on a fact that the vapor pressure in this case is affected by a kneaded resin and shows a value different from a case of the magnesium sulfate single body. Thus, constant humidity to be held was controlled suitably by changing a kind of resin.
That is, the description of the above Patent document 3 is based on an idea of obtaining the composition and the molding having different equilibrium humidity by changing the kind of resin by utilizing the fact that moisture permeability changes depending on the kind of resin. Therefore, the above Patent document 3 did not take the specific gravity of the resin itself into consideration in any way. This is probably based on the following reasons.
Generally, the amount of gas permeation is determined by the following formula: (amount of gas permeation)=(gas permeation coefficient)×(gas pressure difference)×(area)×(time)÷(film thickness). A gas permeation rate (moisture permeability) is obtained by measurement made by fixing the pressure difference, the area, the time, and the film thickness in this formula. The gas permeation rate is a value which changes depending on a gas permeation coefficient, and generally the gas permeation coefficient is determined by the following formula: (gas permeation coefficient)=(diffusion coefficient)×(solubility coefficient). Therefore, when comparison is made by using identical film thickness, the same area, the same time, and the partial pressure difference of the same gas, the amount of gas which moves through a resin film (amount of gas permeation) is decided by the product of the easiness of incorporation of the gas into the film (solubility coefficient), and the easiness of movement of the gas within the film (diffusion coefficient).
If the kind of gas is decided, the solubility coefficient will not change greatly even when a kind of resin (polymer) changes, but changes drastically according to gases for a predetermined polymeric membrane.
The diffusion coefficient changes drastically according to the kind of resin (polymer) forming the polymeric membrane for the same gas, and has no quantitative relationship with the kind of gas, that is, a molecular size and a molecular weight, in a polymeric membrane of the same kind.
Further, resins having a hydroxyl (—OH) or an amide (—CONH—) in the resins are sensitive to water vapor, although polymers are combined firmly together by hydrogen bonds.
When water enters such resins, the hydrogen bonds are destroyed and lost, and as a result, the intermolecular force of the resins is extremely weakened. That is, resin chains are plasticized by the water, gas permeability is increased, and the movement of the resin chains is also activated. Therefore, the gas easily diffuses. Table 1 shows how the gas permeability is increased, when polymers sensitive to the water absorb the water (an extract from “Food Packaging Handbook” published by the Japan Packaging Institute). On the other hand, polymers having a hydrophobic structure (polyethylene and polypropylene) and polymers having low polarity (PVC, PVCD and PET), polymers having a low gas permeation properties due to dipole interactions (PAN), or the like have a smaller amount of water absorption, and gas permeability does not change even when the polymers contain water.
TABLE 1O2 permeation rateResinsConditionsat 25° C.(Note 1)Polyvinyl alcoholDry0.06Polyvinyl alcoholRH 95%310Cellophane (uncoated product)Dry2RH 100%3,110Nylon 6Dry18Nylon 6RH 100%78Eval F(Note 2)Dry0.26Eval FRH 100%31PEDry7,510PERH 100%7,510PolyacrylonitrileDry1.6PolyacrylonitrileRH 100%1.7(Note 1)O2 permeation rate: cc · 25.4 μ/m2 · 24 hr/atm(Note 2)Eval F is a copolymer of ethylene and vinyl alcohol, wherein vinyl alcohol content is approximately 70%.
Table 2 shows O2 and H2O permeation rate of resins generally used currently (an extract from “Food Packaging Handbook” published by the Japan Packaging Institute).
As clearly shown in the table, there is no relationship between the O2 permeation rate and the H2O permeation rate which is generalized in any way.
In the case of gas permeation rate, diffusion is a control factor, and in the case of water vapor permeation rate, the affinity between the water and the resin is a control factor. That is, hydrophobic resins always have small water vapor permeation rates, and hydrophilic resins always have large water vapor permeation rates.
TABLE 2Permeation rate at 25° C.(Note 1)PolymersO2H2OPolyvinyl alcohol (dry)0.065Polyvinyl alcohol (wet)3101,097 (RH 95%)Eval (dry)0.3Eval (wet)3138Polyacrylonitrile (crystallized)0.655.5PVDC (homopolymer)1.60.07PVDC (copolymer)40.5Cellophane (dry)2Polyacrylonitrile copolymer (70%1619AN)Nylon 6 (dry)18Nylon 6 (wet)7847PET (biaxially-stretched film)475PET (biaxially-oriented bottle)788Polychloro trifluoroethylene471.6Nylon 6-10 (dry)93Nylon 6-10 (wet)15522PVC (hard)1245.5PVC (for bottles)1687Polyacetal15547Polymethylmethacrylate26041PET copolymer (PRTG)41426Polyvinyl acetate (dry)910SAN (AN: 25%; ST: 75%)1,01044ABS1,55033High-density PE(d = 0.955)1,7100.5PP2,3301.6Polystyrene6,73030Low-density polyethylene (d = 0.92)7,5102.7Teflon7,7701.2Polybutadiene38,84058Poly-4-methylpentene-162,14047Silicone elastomer1,398,060823(Note 1)O2: cc · 25.4 μ/m2 · 24 hr · atm, RH 65% H2O: g/m2 · 24 hr · atm
As described above, the gas permeation rate, especially the water vapor permeation rate, is a value which changes according to the kinds of gases and resins. Thus it is understood that the value is totally unrelated to the difference in specific gravity of the resins. For this reason, when conventionally obtaining compositions and moldings having different equilibrium humidity, the specific gravity of the resins was probably not taken into consideration.
Further, technical information of a resin manufacturer (Sumitomo Chemical Co., Ltd., “Moisture vapor permeation rate of polyethylene film”, PE technical information III-4{circumflex over (1)}) teaches, as shown in FIG. 18, for example, (sample: LDPE 60 μ-single layer film; measurement conditions: 40° C., RH 90%), that it may be roughly guessed that, when a difference in specific gravity within the same resin is considered, a resin having larger specific gravity has smaller moisture permeation rate. However, in the conventionally common technical knowledge, it was considered that “it is not necessary to take the specific gravity into consideration since the difference in moisture permeation rate due to the difference in the specific gravity is very small”, and there is no definite relational expression for the specific gravity difference in the same resin, and no major difference arises from the specific gravity difference, and accordingly, the specific gravity of the resins was hardly considered.
Under such circumstances, only one equilibrium humidity could be set for one kind of resin in the conventional art, and it was necessary to change the kind of resin for changing constant humidity (equilibrium humidity) to be maintained.
However, manufactured articles acquired different gas barrier properties, physical properties, or the like as a result of changing the kind of resin, and, in many cases, it was difficult to secure an aesthetic property required for products to be stored, or too complicated processing methods adversely affect cost and techniques, which causes a problem of difficulty to form a suitable storage environment for increasingly diversified products.